[release-branch.go1.5] multipart: fixes problem parsing mime/multipart of certain lengths

When parsing the multipart data, if the delimiter appears but doesn't
finish with -- or \n or \r\n, it assumes the data can be consumed. This
is incorrect when the peeking buffer finishes with --delimiter-

Fixes #12662

Change-Id: I329556a9a206407c0958289bf7a9009229120bb9
Reviewed-on: https://go-review.googlesource.com/14652


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/16969


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>